The purpose of the proposed project is to continue the work already started to identify and characterize the cell receptor for bovine leukemia virus (BLV). This study is important as indicated by the following facts: BLV is a naturally occurring infectious retrovirus -- the etiological agent of bovine leukemia. BLV together with human T-cell leukemia viruses (HTLV-I, HTLV-II) belongs to a distinct family of lymphotropic, transactivating retroviruses which share similar biological and pathological properties and molecular mechanisms of action. BLV-related sequences (gag, env) were found integrated in human DNA. The host range and cellular tropism of retroviruses are determined by the presence of receptors on the surface of cell membranes. These biologically important molecules are also involved in virus interference, which is known to play an important role in retroviral infection. So far very little is known about the cell receptors for retroviruses, despite the broad host range of many retroviruses including BLV. The only highly studied receptor is the CD4 antigen, which is the high affinity receptor for human and simian immunodeficiency viruses (HIV, SIV). Understanding retroviral cell receptors has implications not only in virus infectivity; it also has practical importance because of the potential use of this receptor protein in treatment of retroviral infections, for vaccine development, etc. The proposed work will be performed in two basic directions -- on the level of gene and on the level of protein. 1. The gene for the cellular BLV receptor will be isolated by screening of a cDNA expression library from permissive cells. For screening, a cocktail of monoclonal antibodies directed against envelope glycoprotein gp51 will be used. The monoclonal antibodies were developed earlier in the Altaner laboratory. They will be used to visualize plaques that bind BLV gp51. Preliminary experiments already performed have shown the feasibility of this approach to screen the library. The obtained genes will be sequenced, compared with Genbank sequences and further characterized. The binding site will be localized by analysis of products obtained from recombinant DNAs with directed deletions. Precise mapping of the binding site will be performed by means of anti-peptide antisera. Attempts will be to transfect the receptor gene into mammalian cells by use of retroviral vectors to study its biological function. 2. The protein corresponding to the BLV cell receptor will be isolated by means of immunoaffinity chromatography using the highly purified viral glycoprotein gp51. The gp51 of BLV was already isolated by use of monoclonal antibodies. The isolated receptor protein will be characterized, and its function will be studied. The binding site will be mapped with the available battery of monoclonal antibodies.